1. Field of the Invention
The present invention relates to image display devices comprising getters.
2. Description of the Related Art
In image display devices each of which displays images by irradiating a fluorescent material, which functions as an image display member, with electron beams emitted from an electron source so that the fluorescent material emits light, the inside of a vacuum container containing the electron source and the image display member must be placed in a highly evacuated state. In general, the vacuum container of an image display device is formed of glass members bonded together with frit glass or the like which is provided at the bonding portions therebetween, and after the bonding is performed, a pressure inside the vacuum container is maintained by getters which are placed therein.
A getter is a common name of a material which is placed inside a chamber so as to maintain an evacuated state after the chamber being evacuated by a pump or the like. The getter is roughly categorized into an evaporable getter and a non-evaporable getter. The evaporable getter literally forms a metal thin-film on an opposing surface by evaporating a material using high-frequency induction heating, electric heating, or the like so as to suppress the movement of residual gases by chemical reaction (adsorption) thereof with the metal film in an evacuated state, thereby maintaining an evacuated state. In contrast, in the non-evaporable getter mentioned above, a new metal is come out on the getter surface since a metal oxide, carbide, nitride, or the like covering the getter diffuses thereinto by supplying energy thereto by electric heating means or the like, and hence the new metal thus come out becomes able to react with residual gases in an evacuated state, thereby maintaining an evacuated state. In general, a step of exposing a new metal surface is called an activation step, and by this activation step, a getter becomes able to function to maintain an evacuated state. The capabilities of the evaporable and non-evaporable getters for maintaining an evacuated state by reaction with residual gases present in a vacuum are approximately equivalent to each other, and as for the evaporable getter, it is preferable that the distance between the getter and the opposing surface be relatively large in order to form a large surface area of the metal film. In contrast, as for the non-evaporable getter, there has been no distance limitation at all. In addition, as for the non-evaporable getter, when an activation step is again performed after adsorption capability of the getter is fully used, a new metal surface can again be obtained on the surface of the getter since a metal oxide, carbide, nitride, or the like formed on the surface again diffuses into the getter, and hence the getter can be repeatedly used as long as this activation step can be effectively performed. Whether the activation step is effectively performed or not depends on an atmosphere in which the getter is used, and the activation step is preferably preformed in a more highly evacuated state.
In general cathode-ray tubes (CRTs), as the getter described above, an evaporable getter alloy primarily composed of barium (Ba) has been used. A deposition film is formed on inside walls of a CRT, which is sealed beforehand by bonding, by heating an evaporable getter using electricity or high frequency so as to adsorb gases generated inside the CRT, thereby maintaining a highly evacuated state. In CRTs, due to the unique shape thereof, a wall-surface area inside CRT, on which an electron source or an image display member is not provided, is sufficiently present, and on the area described above, a deposition film may be formed by evaporating an evaporable getter.
In addition, in recent years, development of flat display devices has been aggressively performed in which a number of electron emitters functioning as an electron source are disposed on a flat substrate, and electrons generated from the electron source in a vacuum container formed of the electron source and image display member are accelerated by anodes so as to collide against the image display member for displaying images. In the flat display device, a volume of the vacuum container is small compared to that of a CRT; however, a wall-surface area which emits gases is not decreased. Accordingly, when gases are generated having a volume approximately equivalent to that of gases generated in a CRT, a pressure inside the vacuum container is largely increased, and hence the electron source is seriously influenced thereby. In addition, in the case of a flat display device, a large area of the inside walls of the vacuum container is occupied by the electron source and the image display member. Accordingly, when a getter film made from the evaporable getter described above is formed on the area described above, since adverse influences such as short-circuiting of wires may occur, areas in which the getter film is formed are limited to places at which the electron source and the image display member are not provided. For example, it may be considered that a getter film is formed on edge portions inside the vacuum container and is not formed in an area (hereinafter referred to as xe2x80x9cimage display areaxe2x80x9d) which is located between the image display member and the electron source. However, when the size of the flat display device is increased to some extent, it becomes difficult to secure a surface area of the getter film compared to a gas volume which will be generated.
In addition, in the flat display device, a problem in that a pressure is locally increased in the vacuum container may occur in some cases. Parts of the vacuum container at which gases are generated are primarily the image display member irradiated with electron beams and the electron source. In the flat display device, since the image display member and the electron source are close to each other, gases generated from the image display member reach the electron source before being sufficiently diffused, and hence local increase in pressure occurs in the vacuum container. In particular, gases generated at the central portion of the image display area are difficult to diffuse to an area at which the getter film is formed, and hence it has been considered that local increase in pressure frequently occurs at the central portion of the image display area as compared to that at the peripheral portion thereof.
Accordingly, in the flat display device, in addition to the peripheral portion of the image display area, the structure in which a getter material is provided in the image display area so as to adsorb a gas immediately has been considered.
However, when the size of a non-evaporable getter disposed at the periphery of the image display area is increased to some extent, the distance between the getter and an anode plate used for image display is decreased, and as a result, discharge therebetween may occur by a high voltage applied during display operation in some cases. When the discharge occurs, a high voltage at which the discharge occurs cannot be applied, and hence a brighter image cannot be displayed.
In addition, by thermal expansion of the non-evaporable getter which occurs during activation thereof, the getter may be unexpectedly brought into contact with members forming the flat display device, and in some cases, the display itself may be damaged. In order to prevent the problem described above, placement of the getters and constituent members must be performed with high accuracy, and as a result, the yield may be decreased in some cases.
In addition, when a non-evaporable getter is activated by electric heating, terminals for supplying electricity must extend outside the vacuum container, and as a result, a vacuum leak which occurs at the terminals may decrease the yield in some cases.
Furthermore, depending on image quality to be displayed, a vacuum container constituting the display device must be designed so that the height has upper and lower values. Depending on the values of height design, a non-evaporable getter having a conventional volume may not be provided in the vacuum container in some cases. In addition, when a core member of non-evaporable getter is too small, a technical problem in that material of getter cannot be fixed thereto may also occur in some cases.
An object of the present invention is to provide a image display device capable of maintaining image quality while increase in pressure in a vacuum container is prevented and suppressing discharges with an anode plate while display is performed.
In addition, another object of the present invention is to provide an image display device which can be manufactured with a high yield by avoiding damage done to non-evaporable getters provided at the periphery of the image display area and vacuum leaks, which occur during activation.
The present invention relates to an image display device having a container which includes an electron source substrate, an electron source provided thereon, and an image display member which opposes the electron source substrate and which displays an image when being irradiated with electrons emitted form the electron source. The container described above further comprises first getters provided in an image display area which is located between the electron source and the image display member, and ring non-evaporable second getters provided outside the image display area.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.